Externally electroless method for producing a nickel alloy and corresponding electrolyte

ABSTRACT

The invention relates to a method for producing a nickel-phosphorus alloy in which a metallic substrate is dipped into an aqueous electrolyte containing at least nickel cations, hypophosphite ions, stabilisers and complexing agents, characterised in that the electrolyte additionally contains pyrones of formula I 
     
       
         
         
             
             
         
       
     
     or the derivatives or salts thereof, wherein R1 represents a hydrogen atom or a hydroxyl group, R2 a methyl group, an ethyl group or a hydroxymethyl group, R3 a hydrogen atom or a hydroxyl group, and R4 a hydrogen atom, a hydroxyl group or a methyl ketone group, and to a corresponding electrolyte.

CROSS REFERENCE

This application claims priority to German Patent Application 10 2017125954.6, filed Nov. 7, 2017, the entirety of which is herebyincorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a chemical, i.e. externally electrolessmethod for producing a nickel alloy by electroless metal deposition inan aqueous electrolyte and to a corresponding electrolyte.

BACKGROUND

Nickel phosphorus is generally deposited chemically as corrosion andwear protection on metallic materials in the form of a nickel coating.In contrast to nickel electrodeposition, no electricity flow is used fordeposition.

These nickel alloys comprise a nickel phosphorus alloy which is used inparticular in functional areas of use. The phosphorus deposited in thelayer may be used to control the layer properties. In this case, adistinction is drawn between a high (10-14 wt. %), medium (9-12 wt. %)and low (3-7 wt. %) phosphorus content.

A method is known from EP 1 330 558 B1, for example, for producing alead-free nickel alloy in which a metallic substrate is dipped into anaqueous electrolyte containing nickel cations, hypophosphite ions andbismuth ions in a concentration of bismuth of at most 0.3 ppm andantimony ions in a concentration of antimony of at least 10 ppm.

An electrolyte for producing lead-free, externally electrolesslyproduced nickel layers is known from U.S. Pat. No. 2,884,344. Accordingto the method described therein, at least two cations are added to theelectrolyte, these being selected from the group comprising antimony,arsenic and bismuth.

This autocatalytic deposition of nickel and co-deposition of phosphorusin the presence of sodium hypophosphite to produce nickel-phosphorusalloys has developed in the last few decades from a niche product to amajor method used in the electroplating industry.

Due to the co-deposited phosphorus, the resultant layers exhibitconsiderably improved corrosion resistance and hardness.

Owing to their excellent corrosion resistance (requirements), nickelalloys which have a particularly high phosphorus content are desired.

SUMMARY OF THE INVENTION

Provided herein is a method for producing a nickel-phosphorus alloywherein a metallic substrate is dipped into an aqueous electrolytecomprising nickel cations, hypophosphite ions, at least one stabiliser,and at least one complexing agent, and wherein the electrolyte furthercomprises a pyrone of formula I

or a derivative or salt thereof, wherein R1 is selected from the groupconsisting of a hydrogen atom and a hydroxyl group, R2 is selected fromthe group consisting of a methyl group, an ethyl group, and ahydroxymethyl group, R3 is selected from the group consisting of ahydrogen atom and a hydroxyl group, and R4 is selected from the groupconsisting of a hydrogen atom, a hydroxyl group, and a methyl ketonegroup.

Also provided herein is an aqueous electrolyte for producingnickel-phosphorus alloys comprising nickel cations, hypophosphite ions,at least one stabiliser, and at least one complexing agent, and furthercomprising a pyrone of formula I

or a derivative or salt thereof, wherein R1 is selected from the groupconsisting of a hydrogen atom and a hydroxyl group, R2 is selected fromthe group consisting of a methyl group, an ethyl group, and ahydroxymethyl group, R3 is selected from the group consisting of ahydrogen atom and a hydroxyl group, and R4 is selected from the groupconsisting of a hydrogen atom, a hydroxyl group, and a methyl ketonegroup.

DETAILED DESCRIPTION OF THE INVENTION

The object of the invention is therefore that of providing a method forproducing an externally electrolessly produced nickel alloy which bringsabout a particularly high rate of incorporation or co-deposition ofphosphorus. A further object consists in providing an electrolytesuitable therefor.

It has been recognised according to the invention that by using anelectrolyte additionally containing pyrones of formula I

or the derivatives or salts thereof (which liberate the correspondingcompound in the aqueous medium), wherein R1 represents a hydrogen atomor a hydroxyl group, R2 a methyl group, an ethyl group or ahydroxymethyl group, R3 a hydrogen atom or a hydroxyl group, and R4 ahydrogen atom, a hydroxyl group or a methyl ketone group, the methodsleads to high levels of phosphorus deposition and thus tonickel-phosphorus alloys with a high phosphorus content.

In other words, nickel-phosphorus alloys which have a phosphorus contentof greater than 10 wt. % based on all the components of the alloy may beproduced according to the invention. Such a high deposition rate ispreferably achieved in that a phosphorus content of greater than 11 wt.% based on all the components of the alloy is achieved.

The pyrones of formula I may thus comprise inter alia ethyl maltol,kojic acid or dehydroacetic acid (DHA).

In a particularly preferred embodiment, R1 represents a hydroxyl group,R2 a ethyl group and R3 and R4 a hydrogen atom in a pyrone of formula I.In other words, the use of ethyl maltol as an addition is particularlypreferred and particularly good results in terms of phosphorus contentsare achieved with this pyrone. Phosphorus contents in the alloy of over11 wt. % may thereby be achieved.

It has furthermore been found that if the electrolyte additionallycontains ascorbic acid, iso-ascorbic acid or the salts thereof, inaddition to a high phosphorus incorporation rate it is also possible toaccelerate deposition of the phosphorus, and this without the assistanceof sulfur-containing compounds or further semimetals, such as seleniumand tellurium. In particular, a deposition rate of generally more than12 μm, specifically more than 16 μm and ideally more than 20 μm per hourmay be achieved.

The addition of ascorbic acid or iso-ascorbic acid or the salts thereofdoes not in any way impair stability. On the contrary, ascorbic acidstabilises the system, as may be expected of a bidentate ligand. On theother hand, ascorbic acid accelerates deposition in a similar way assulfur compounds are known to do.

Alternatively, the known sulfur compounds may be used.

The two types of acceleration are not mutually exclusive and may becombined, i.e. sulfur-containing compounds of known type may also beused in addition to ascorbic acid. It is therefore possible toaccelerate medium phosphorus methods to up to 40 μm per hour and highphosphorus methods to over 20 μm per hour.

It has likewise been found that if the electrolyte additionally containsa mono-hydroxycarboxylic acid or the salts thereof, a furtherimprovement in phosphorus content may be achieved while maintaining thedeposition rate.

It is preferable for the pyrones of formula Ito be present in an amountof between 5 ppm and the solubility limit, preferably between 0.1 g/land 25 g/L, particularly preferably between 0.5 g/L and 20 g/L.

If the method is performed at an electrolyte temperature of at least 85°C., preferably at least 92° C., a marked increase in deposition rate maybe achieved. This increase does not proceed continuously, but rather instages. That is to say, three discrete temperature ranges form: (i)below 85° C. virtually no deposition takes place, as with the knownmethods, (ii) between 86 and 91° C. 50 to 70% faster deposition takesplace compared with traditional methods and (iii) from 92° C. thedeposition rate is at least doubled.

Moreover, representatives (for example ethyl maltol, kojic acid, DHA) ofthe stated substance class have pK values of between 5 and 10, whichmakes it possible to increase the pH of the coating solution to over 5without increasing the concentration of free nickel ions in aqueoussolution. This increase in pH promotes the reducing properties of thehydrophosphite, whereby the deposition rate together with the ascorbicacid is more than doubled overall without impairing phosphorusincorporation.

A particularly high deposition rate is achieved when the pH is increasedto over 5 with simultaneous use of ascorbic acid.

The present invention accordingly also provides an aqueous electrolytefor producing nickel-phosphorus alloys containing at least nickelcations, hypophosphite ions, stabilisers, complexing agents and reducingagents, characterised in that the electrolyte additionally containspyrones of formula I

or the derivatives or salts thereof (which liberate the correspondingcompound in the aqueous medium), wherein R1 represents a hydrogen atomor a hydroxyl group, R2 a methyl group, an ethyl group or ahydroxymethyl group, R3 a hydrogen atom or a hydroxyl group, and R4 ahydrogen atom, a hydroxyl group or a methyl ketone group.

EXAMPLES

The following Examples illustrate the invention.

One litre of an aqueous solution consisting of 5 g/L Ni²⁺ cations,introduced as nickel sulfate, 40 g/L sodium hypophosphite monohydrate,35 g/L iso-ascorbic acid, 45 g/L lactic acid, 35 g/L ethyl maltol, 2 g/Lsodium saccharin and 1 ppm Pb²⁺ cations, introduced as lead acetate,were heated to 93° C. at a pH of 4.5 (adjusted with sodium carbonate).

A small (special) steel plate with a total surface area of 1 dm²,previously cleaned for 15 minutes by conventional commercial heatdegreasing with ultrasound and then activated for 4 minutes in a 10 percent hydrochloric acid solution, was coated therein for one hour. Thesmall plate was weighed, the layer with semi-concentrated nitric acidwas etched away, the weight was determined again and the nickel andphosphorus content in the etching solution was determined using ICP-OES(inductively coupled plasma atomic emission spectroscopy; instrumentmanufactured by Agilent Technologies (720 Series)) to DIN ISO 4527.

At a deposition rate of 17 μm per hour, the layer exhibited a P contentin the alloy of 11.2 wt. % (based on all the components of the alloy).

A comparative example without hydroxy-gamma-pyrone compound with asolution consisting of 5 g/L Ni²⁺ cations, introduced as nickel sulfate,40 g/L sodium hypophosphite solution, 22 g/L succinic acid, 45 g/Llactic acid, 5 g/L sodium saccharin and 1 ppm Pb²⁺ cations, introducedas lead acetate, resulted, at a pH of 4.5 and 93° C., in a depositionrate of 12 μm per hour and a P content in the alloy of 8.4 wt. %.

As in the above example, in further exemplary embodiments 2 to 9 onelitre of an aqueous solution was heated which contained the contents ofadditives shown in Table 1 below.

TABLE 1 Sodium Kojic Lactic Malic Ethyl Sodium hypophosphite DHA acidacid acid maltol saccharin monohydrate Nickel No. (g/L) (g/L) (g/L)(g/L) (g/L) (g/L) (g/L) (g/L) 2 3.0 0.2 50 — — 5 40 5 3 3.0 0.6 30 12.0— 5 40 5 4 3.0 — 50 — — 5 40 5 5 — 4.0 50 — — 5 40 5 6 3.0 6.0 50 — — 540 5 7 2.0 1.0 32 11.9 — 5 40 5 8 3.5 6.0 50 — 1.0 5 40 5 9 3.0 6.0 50 —— 5 40 5A small (special) steel plate was coated therein in accordance with theabove details and then examined.

The pH and temperature values established in these tests are listed inTable 2, together with the results obtained, namely the phosphoruscontent (P content in the alloy) and deposition rate.

TABLE 2 Temp. Deposition rate P content No. (° C.) pH (μm/h) (weightpercent) 2 92 5.5 16.8 >11 3 92 5.5 18.0 >11 4 90 5.0 17.1 >11 5 90 5.016.9 >11 6 90 5.0 17.2 >11 7 90 5.0 15.2 >11 8 95 5.0 17.3 >11 9 95 5.323.1 >11

1. A method for producing a nickel-phosphorus alloy wherein a metallicsubstrate is dipped into an aqueous electrolyte comprising nickelcations, hypophosphite ions, at least one stabiliser, and at least onecomplexing agent, and wherein the electrolyte further comprises a pyroneof formula I

or a derivative or salt thereof, wherein R1 is selected from the groupconsisting of a hydrogen atom and a hydroxyl group, R2 is selected fromthe group consisting of a methyl group, an ethyl group, and ahydroxymethyl group, R3 is selected from the group consisting of ahydrogen atom and a hydroxyl group, and R4 is selected from the groupconsisting of a hydrogen atom, a hydroxyl group, and a methyl ketonegroup.
 2. The method of claim 1 wherein R1 is a hydroxyl group, R2 is anethyl group, R3 is a hydrogen atom, and R4 is a hydrogen atom.
 3. Themethod of claim 1 wherein the electrolyte further comprises at least oneascorbic acid compound selected from the group consisting of L-ascorbicacid and salts thereof, iso-ascorbic acid and salts thereof, andsubstances that produce L-ascorbic acid or iso-ascorbic acid in theelectrolyte.
 4. The method of claim 1 wherein the electrolyte comprisesthe at least one ascorbic acid compound in an amount between 0.5 g/L and100 g/L.
 5. The method of claim 1 wherein the electrolyte furthercomprises a mono-hydroxycarboxylic acid or a salt thereof.
 6. The methodof claim 1 wherein the compound of formula I is present in aconcentration of at least 5 ppm.
 7. The method of claim 1 wherein thecompound of formula I is present in a concentration of between 0.1 g/land 25 g/L.
 8. The method of claim 1 wherein the method is carried outat an electrolyte temperature of at least 85° C.
 9. The method of claim7 wherein the method is carried out at an electrolyte temperature of atleast 92° C.
 10. An aqueous electrolyte for producing nickel-phosphorusalloys comprising nickel cations, hypophosphite ions, at least onestabiliser, and at least one complexing agent, and further comprising apyrone of formula I

or a derivative or salt thereof, wherein R1 is selected from the groupconsisting of a hydrogen atom and a hydroxyl group, R2 is selected fromthe group consisting of a methyl group, an ethyl group, and ahydroxymethyl group, R3 is selected from the group consisting of ahydrogen atom and a hydroxyl group, and R4 is selected from the groupconsisting of a hydrogen atom, a hydroxyl group, and a methyl ketonegroup.
 11. The aqueous electrolyte of claim 10 wherein the electrolytefurther comprises at least one ascorbic acid compound selected from thegroup consisting of L-ascorbic acid and salts thereof, iso-ascorbic acidand salts thereof, and substances that produce L-ascorbic acid oriso-ascorbic acid in the electrolyte.
 12. The aqueous electrolyte ofclaim 11 wherein the electrolyte comprises the at least one ascorbicacid compound in an amount between 0.5 g/L and 100 g/L.
 13. The aqueouselectrolyte of claim 10 wherein the electrolyte further comprises amono-hydroxycarboxylic acid or a salt thereof.
 14. The aqueouselectrolyte of claim 10 wherein the compound of formula I is present ina concentration of at least 5 ppm.
 15. The aqueous electrolyte of claim10 wherein the compound of formula I is present in a concentration ofbetween 0.1 g/l and 25 g/L.
 16. The aqueous electrolyte of claim 10comprising a compound of formula I wherein R1 is a hydroxyl group, R2 isan ethyl group, R3 is a hydrogen atom, and R4 is a hydrogen atom.